Microwave oven transformer welder (inverter)?

A Microwave Oven Power Supply (MOPS) of any kind is a bad choice to make any kind of welder. A MOPS is designed to produce a lot of power at high voltage. A welding power supply produces a very tiny amount of power at a high voltage and then when the arc strikes, a low voltage high current power supply takes over to make the weld. Stay away from a MOPS, they are extremely dangerous. The article you pointed to was about using a MOPS to make a very high power RF transmitter, which is nothing like a welding power supply. There are plenty of very cheap home welding units on the market. buy one of those.